Wind turbines are normally controlled in accordance with a control strategy based on one or more measured control parameters, such as wind speed, wind direction, etc. Accordingly, a control system used for controlling operation of a wind turbine is normally connected to one or more sensors, each sensor being arranged to measure a specific control parameter.
In most cases the control parameters are measured by means of separate sensors measuring quantities which do not have a direct impact on the components of the wind turbine. For instance, wind speed is often measured by means of a wind speed sensor, such as a cup anemometer or an ultrasonic wind sensor, positioned on top of the nacelle of the wind turbine or at a separate position in the vicinity of the wind turbine. Such a wind speed sensor measures the wind speed in a single point, and therefore such a measurement is not necessarily representative of the wind speed occurring in the wind field of the entire rotational plane of the rotor blades of the wind turbine. Furthermore, in the case that the wind speed sensor is arranged on top of the nacelle, the measured value of the wind speed may be influenced by disturbances in the wind field caused by the rotor blades. Accordingly, relying on such measured values of the wind speed when controlling the operation of the wind turbine introduces a certain level of unreliability, thereby making it difficult, or even impossible, to control the wind turbine optimally.
It is therefore desirable to be able to control the wind turbine based on control parameters obtained in a more reliable manner.
It is sometimes difficult or impossible to obtain reliable values of a preferred control parameter. This may, e.g., be because the sensor used for measuring the control parameter is out of order, or because the operating conditions are such that the sensor does not operate reliably, e.g. due to fog, heavy rain or snow, ice formation, etc. In this case it is desirable to be able to control the operation of the wind turbine in accordance with a control strategy based on another control parameter which can be measured in a reliable manner.
GB 2 067 247 discloses a wind turbine having pressure probes mounted at the surface of the rotor blades for ascertaining the wind energy in the rotor plane. The indications from the pressure probes are used during normal operation of the wind turbine. An anemometer is placed on top of the nacelle for measuring wind speed on top of the nacelle. The anemometer is used for control of the wind turbine outside the operating range of the turbine, when the turbine is on standstill due to absence of wind or in case of a storm. However, the anemometer suffers from the drawbacks described above, i.e. it only measures the wind speed in one point and the measurement may be disturbed by the rotating rotor blades. Accordingly, measured values from the anemometer can not be used for controlling the wind turbine in accordance with a control strategy requiring precise information regarding physical impact on the wind turbine.
WO 2006/069573 discloses a wind turbine comprising equipment under control. The wind turbine comprises at least one control system for one or more of the main components of the wind turbine. The control system is multiplied by at least one further control system for controlling the same of said equipment under control. It is possible to switch between using the first and the second control system, and redundancy is thereby obtained. However, the control systems monitor same the control parameter(s), and the applied control strategy is therefore not changed when a switch from one control system to another is performed, i.e. the operation of the wind turbine is controlled on the basis of a given control parameter, regardless of which of the control systems is used.